Studying genitives, specifically partitives, in my on-again off-again torturous journey through Revelation (but I've learned a lot doing it). So I came across a passage I've had trouble with before, but for a different reason this time.

Simple definition of partitive genitive: The head substantive in the phrase represents a "part" of some whole, which is described by the genitive.

The definition that you're using only applies to genitives that modify substantives. It doesn't cover genitive objects of prepositions. Both ἐκ and ἀπό can be used to express partitive relationships, but most often the genitive is the whole and the part is the substantive that is "from" (ἐκ/ἀπό) the whole.

Thanks, I guess that explains why I was having trouble with it. So, just to be clear, τῶν ἑπτά represents the "whole," and in this instance ἐκ is the "part"? Would this explain why some versions translate it "one of the seven," in order to supply a head noun for what ἐκ implies?